To those skilled in the art of satellite navigation systems, the term “integrity” refers to the degree of confidence that a user of a satellite navigation system can have that the position information provided by the system is “correct”; that is, it is within acceptable limits. The maximum tolerable error in such position information is called an “alert limit” (including a horizontal alert limit and a vertical alert limit), and the term “integrity risk” refers to the probability that the system will fail to provide a warning within a specified time period after a given alert limit has been exceeded.
The requirements for integrity performance (and the associated horizontal and vertical alert limits) vary as between several different phases of flight, such as APV (approach with vertical guidance) APV I, APV II, CAT I (category I precision approach), oceanic, and en route/terminal. The table set forth below shows typical examples:
TABLE IHorizontal/VerticalnumberPhase of FlightIntegrity RiskAlert Limit [m]1Oceanic en-route   10−7/hour7412/— 2En-route/Terminal   10−7/hour556/— 3APV2 × 10−7/150 sec556/50 4APV I2 × 10−7/150 sec40/205APV II2 × 10−7/150 sec40/106CAT I   10−9/15 sec20/10
The specification for the satellite navigation system Galileo demands only an integrity service with a specified integrity risk for fixed horizontal and vertical alarm limits. However, for applications in the area of flight safety the subscribed users require a concept that demands different integrity requirements in different phases of the flight (such as set forth above). In addition, the user groups are not satisfied with knowing the integrity risk at the alarm limit, but rather demand so-called protection levels.
The prior art does not address the problem of different integrity risks for different integrity requirements. Furthermore, there do not exist any solutions that are optimal for alert limits that are larger than the smallest set of alert limits.
One object of the present invention is to provide a method and apparatus that make it possible for a receiver to calculate the integrity risk, without having to know the momentary flight phase.
This object is achieved by a method for determining protection levels in a satellite navigation system which comprises the following steps: (1) determining an integrity risk at the alert limits for a variety of flight phases or situations, (2) determining an interval of the alert limits, between the largest set of alert limits and the integrity risk which produces too high an integrity risk, and the smallest set of alert limits and the integrity risk which produces an acceptable integrity risk; and (3) carrying out an interval nesting for the interval of the alert limits that was determined in the previous step. In this case the integrity risk between the horizontal and the vertical is divided in the same way as it is obtained from the relationship between these integrity risks in the largest set of alert limits.
When the integrity risk is determined in this manner, by the method, according to the invention, it is no longer necessary to know the momentary flight phase, because the protection level is always determined in such a way that when the protection level for a flight phase has met the requirements of Table 3.7.2.4-1 in Part 1 of the Annex 10 to the ICAO Convention (International Civil Aviation Organization Convention), the integrity risk is correctly selected. Due to the choice of the division, the smallest possible alert limit is also always determined. Due to the interval nesting it is guaranteed that the integrity risk is always conservative. And due to the interval nesting, smaller conservative alert limits can be determined through the use of better processors in the receiver.
The invention also includes a device for determining the protection levels in a satellite navigation system, which includes elements for performing the method steps described above. As mentioned, the integrity risk between the horizontal and the vertical is divided in the same way as it is obtained from the relationship between these integrity risks in the largest set of alert limits.
Since the device determines the integrity risk in this manner, by the method according to the invention, it is no longer necessary for the device to know the momentary flight phase; because the protection level is always determined in such a way that when the protection level for a flight phase has met the requirements of Table 3.7.2.4-1 in Part 1 of the Annex 10 to the ICAO Convention, the integrity risk is correctly selected. Due to the choice of the division, the smallest possible alert limit is also always determined. Due to the interval nesting, the integrity risk is always conservative. And due to the interval nesting it is also achieved that smaller conservative alert limits can be determined through the use of better processors in the receiver.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.